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They further suggested that labA and sasA function in parallel to regulate KaiBC expression. Upon
interaction with KaiC, SasA gets autophosphorylated and activates RpaA which in turn controls
circadian gene expression. RpaA is considered to be a transcription factor that controls global gene
expression through basic transcription machinery. The SasA-RpaA pathway seems to be activated
by Kai proteins during the subjective day. It is further suggested that KaiC represses RpaA function
through the LabA-dependent pathway that is operative in the subjective night phase.
Katayama et al . (2003) identifi ed ldpA (light-dependent period A) gene that encodes a new
component of the input pathway. In contrast to cikA -defi cient mutants, inactivation or defi ciency of
ldpA still can reset the clock in response to a dark pulse. Based on electron paramagnetic resonance
spectra, the existence of two Fe 4 S 4 clusters have been shown in the gene product of ldpA suggesting
the involvement of LdpA in the regulation of the circadian period by sensing light-dependent
changes in electron transport. Further the existence of two redox-active centers consistent with two
[Fe 4 S 4 ] 2+/1+ clusters in LdpA and the requirement of LdpA for light-dependent modulation of cikA
have a bearing in sensing redox reactions. Apart from this, LdpA copurifi es with Kai proteins that
are integral part of the circadian clock.
V. OUTPUT PATHWAYS
Tsinoremas et al . (1994) identifi ed a gene that is part of the specifi c output pathway of the clock in
S . elongatus PCC 7942. By following the procedure of insertional inactivation of genes developed by
Dolganov and Grossman (1993), a number of mutants that showed short period (M2), arrhythmic
(M11) and low amplitude oscillation of bioluminescence (M16) have been subsequently isolated
and characterized (Tsinoremas et al ., 1994). Further studies revealed that these mutants not only
exhibited defects in the genes governing circadian period (since they resembled mutants developed
after methylmethane sulfonate mutagenesis described above) but also were shown to be defective
in newly identifi ed clock ORFs. Tsinoremas et al . (1996) reported that a mutation in the rpoD2 gene
that encodes a group 2 σ 70 -like transcription factor causes a low amplitude phenotype in the psbAI
gene expression rhythm. These workers proposed a model in which temporal incorporation of the
RpoD2 sigma factor into RNA polymerase decreases the expression of psbAI during subjective night
and generates a high amplitude circadian oscillation of psbAI expression. Further the specifi city
of the mutation for a subset of genes indicates that the rpoD2 gene is a component of the output
pathway in the Synechococcus circadian clock. Another gene ndhD regulates a similar phenotype but
the expression of rhythms by several other genes was not affected by the mutation. Inactivation of
rpoD2 decreases the amplitude of the expression rhythm not only of ndhD but also that of psbAI .
However, it had no affect on the expression rhythm of purF that encodes the oxygen-sensitive enzyme
glutamine phosphoribosyl pyrophosphate amidotransferase (glutamine PRPP amidotransferase),
a key regulatory enzyme in the purine biosynthetic pathway. Katayama et al . (1999) postulated that
the activities of enzymes that are specifi cally expressed in the night, like purF are inhibited by the
approach of the peak of a subset of class 1 rhythms of photosynthetic genes psbAI and psbAII into
their phase.
The genetically engineered strain of S. elongatus PCC 7942 possessing psbAI -promoter belongs to
Class 1` gene family exhibiting peaks at dusk period and troughs at dawn. In contrast Class 2 genes
such as purF shows peaks and troughs at dawn and dusk, respectively. Johnson and Golden (1999)
are of the view that the Class 2 gene expression pattern of purF gene is related to oxygen sensitivity
of glutamine PRPP amidotransferase and it is similar to the temporal separation of nitrogenase in
certain nitrogen-fi xing cyanobacteria described earlier (Liu et al ., 1996; Johnson et al ., 1998).
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